Paint Blistering

I am having trouble painting our garden furniture.I sanded it down but not to the bare wood as there are several coats of paint.I got it nice and smooth then applied two coats of primer with two hours between the coats and then two coats of Cuprinol exterior paint which looked fine when it was dry.But the next morning there were lots of blisters which then disapeared later in morning it must have been the dew during the night.One day last week I painted a bench but later in the day we had some rain and that caused the paint to be blister.Any advice appreciated.